Mirrored object doesn't match it perfectly?
It looks like the origin of the mesh is not quite in the correct place. 


Select the vertices that you wish to be the centre of your mirror (the vertical line of vertices which run from the nose all the way
back around the head and back up to the nose)
Press Shift+S and choose Cursor to Selected.
Go into object mode and choose Origin> Origin to 3D cursor from the Toolshelf (press T to show the toolshelf).


The mirror modifier should then be moved to be above the subsurf modifier to avoid having a visible seam.

Update: To make the centre line of vertices completely straight, select the centre loop and press S , Y then 0.